Modulation of Signal Field in a WLAN Frame Header

ABSTRACT

A plurality of training fields of a preamble of a physical layer data unit are generated, and training fields of the plurality of training fields are mapped to respective signal streams of a plurality of streams using a mapping matrix. A signal field of the preamble of the data unit is generated, and the signal field is mapped to the plurality of signal streams using a column of the mapping matrix. The plurality of signal streams are mapped to a plurality of spatial streams, and at least i) the plurality of training fields and ii) the signal field are transmitted via the plurality of spatial streams.

CROSS-REFERENCES TO RELATED APPLICATIONS

This disclosure claims the benefit of U.S. Provisional Patent Application No. 61/360,828, filed on Jul. 1, 2010, which is hereby incorporated by reference herein in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to communication networks and, more particularly, to communicating device capabilities between devices in a wireless network.

BACKGROUND

The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

Development of wireless local area network (WLAN) standards such as the Institute for Electrical and Electronics Engineers (IEEE) 802.11a, 802.11b, 802.11g, and 802.11n Standards, has improved single-user peak data throughput. For example, the IEEE 802.11b Standard specifies a single-user peak throughput of 11 megabits per second (Mbps), the IEEE 802.11a and 802.11g Standards specify a single-user peak throughput of 54 Mbps, and the IEEE 802.11n Standard specifies a single-user peak throughput of 600 Mbps. Work has begun on a new standard, IEEE 802.11ac, that promises to provide even greater throughput.

SUMMARY

According to one embodiment, a method includes generating a plurality of training fields of a preamble of a physical layer data unit and mapping training fields of the plurality of training fields to respective signal streams of a plurality of streams using a mapping matrix. The method also includes generating a signal field of the preamble of the data unit and mapping the signal field to the plurality of signal streams using a column of the mapping matrix. The method further includes mapping the plurality of signal streams to a plurality of spatial streams and transmitting at least i) the plurality of training fields and ii) the signal field via the plurality of spatial streams.

According to another embodiment, an apparatus includes a network interface, including a physical layer processing unit configured to generate a plurality of training fields of a preamble of a physical layer data unit and map training fields of the plurality of training fields to respective signal streams of a plurality of streams using a mapping matrix. The physical layer processing unit is also configured to generate a signal field of the preamble of the data unit and map the signal field to the plurality of signal streams using a column of the mapping matrix. The physical layer processing unit is further configured to map the plurality of signal streams to a plurality of spatial streams and cause at least i) the plurality of training fields, and ii) the signal field to be transmitted via the plurality of spatial streams.

According to another embodiment, a method includes generating a plurality of training fields of a preamble of a multi-user physical layer data unit and mapping training fields of the plurality of training fields to respective signal streams of a plurality of streams using a mapping matrix, wherein different portions of each column of the mapping matrix correspond to different client devices including a first client device and a second client device. The method also includes generating a first signal field of the preamble of the data unit corresponding to the first client device and mapping the first signal field to the plurality of signal streams corresponding to the first client device using a portion of a column of the mapping matrix corresponding to the first client device. The method further includes generating a second signal field of the preamble of the data unit corresponding to the second client device and mapping the second signal field to the plurality of signal streams corresponding to the second client device using another portion of the column of the mapping matrix corresponding to the second client device. The method still further includes mapping the plurality of signal streams to a plurality of spatial streams and transmitting at least i) the plurality of training fields, ii) the first signal field, and iii) the second signal field via the plurality of spatial streams.

According to another embodiment, an apparatus includes a network interface, including a physical layer processing unit configured to generate a plurality of training fields of a preamble of a multi-user physical layer data unit and map training fields of the plurality of training fields to respective signal streams of a plurality of streams using a mapping matrix, wherein different portions of each column of the mapping matrix correspond to different client devices including a first client device and a second client device. The physical layer processing unit is also configured to generate a first signal field of the preamble of the data unit and map the first signal field to the plurality of signal streams using a portion of a column of the mapping matrix corresponding to the first client device. The physical layer processing unit is further configured to generate a second signal field of the preamble of the data unit corresponding to the second client device and map the second signal field to the plurality of signal streams corresponding to the second client device using another portion of the column of the mapping matrix corresponding to the second client device. The physical layer processing unit is still further configured to map the plurality of signal streams to a plurality of spatial streams and cause at least i) the plurality of training fields, ii) the first signal field, and iii) the second signal field to be transmitted via the plurality of spatial streams.

According to another embodiment, a method includes generating a signal field of a preamble of a physical layer data unit and mapping the signal field to a first plurality of data subcarriers corresponding to a first frequency portion of an orthogonal frequency division multiplex (OFDM) symbol. The method also includes setting a subset of data subcarriers in the first plurality of data subcarriers to one or more predetermined values. The method further includes mapping the signal field to a second plurality of subcarriers corresponding to a second frequency portion of the OFDM symbol and setting a subset of subcarriers in the second plurality of subcarriers to one or more predetermined values. The method still further includes setting guard tones, direct current (DC) tones, and pilot tones in the first frequency portion and the second frequency portion and transmitting the physical layer data unit that includes the OFDM symbol in the preamble of the physical layer data unit.

According to another embodiment, an apparatus includes a network interface, including a physical layer processing unit configured to generate a signal field of a preamble of a physical layer data unit and map the signal field to a first plurality of data subcarriers corresponding to a first frequency portion of an orthogonal frequency division multiplex (OFDM) symbol. The physical layer processing unit is also configured to set a subset of data subcarriers in the first plurality of data subcarriers to one or more predetermined values. The physical layer processing unit is further configured to map the signal field to a second plurality of subcarriers corresponding to a second frequency portion of the OFDM symbol and set a subset of subcarriers in the second plurality of subcarriers to one or more predetermined values. The physical layer processing unit is still further configured to set guard tones, direct current (DC) tones, and pilot tones in the first frequency portion and second frequency portion and cause the physical layer data unit that includes the OFDM symbol in the preamble of the physical layer data unit to be transmitted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example embodiment of a wireless local area network (WLAN) that utilizes various signal field modulation and mapping techniques described herein.

FIG. 2 is a diagram of an example data unit format, according to an embodiment.

FIG. 3 is a block diagram of an example PHY processing unit, according to an embodiment.

FIG. 4 is a diagram of an example OFDM symbol for 40 MHz communication channel that the PHY processing unit of FIG. 3 is configured to generate, according to an embodiment.

FIG. 5 is a diagram of another example OFDM symbol for 40 MHz communication channel that the PHY processing unit of FIG. 3 is configured to generate, according to another embodiment.

FIG. 6 is a diagram of an example OFDM symbol for 80 MHz communication channel that the PHY processing unit of FIG. 3 is configured to generate, according to an embodiment.

FIG. 7 is a diagram of another example OFDM symbol for 80 MHz communication channel that the PHY processing unit of FIG. 3 is configured to generate, according to another embodiment.

FIG. 8 is a flow diagram of an example method for generating and transmitting a PHY data unit having a signal field, such as a VHT-SIGB or another suitable field, according to an embodiment.

FIG. 9 is a flow diagram of another example method for generating and transmitting a PHY data unit having a signal field, such as a VHT-SIGB or another suitable field, according to another embodiment.

FIG. 10 is a flow diagram of an example method for generating and transmitting a multi-user PHY data unit having a signal field, such as a VHT-SIGB or another suitable field, according to an embodiment.

DETAILED DESCRIPTION

In embodiments described below, a wireless network device such as an access point (AP) of a wireless local area network (WLAN) transmits data streams to one or more client stations. The AP is configured to operate with client stations according to at least a first communication protocol (e.g., the IEEE 802.11ac Standard, now in the process of being standardized). Additionally, a different client station in the vicinity of the AP may be configured to operate according to a second communication protocol (e.g., the IEEE 802.11n Standard, the IEEE 802.11a Standard, the IEEE 802.11g Standard, etc.). The first communication protocol is referred to herein as a very high throughput (VHT) protocol, and the second communication protocol is referred to herein as a legacy protocol. In an embodiment, each one of communication protocols (i.e., the legacy protocol and the VHT protocol) defines one or more possible transmission channel bandwidths. Accordingly, a data unit transmitted or received by the AP includes a preamble comprising a legacy portion corresponding to a bandwidth defined in a legacy protocol (e.g., 20 MHz bandwidth defined in the 802.11a protocol) and a VHT portion corresponding to the same or a different channel bandwidth defined in the VHT protocol (e.g., 80 MHz bandwidth defined in the VHT protocol), according to an embodiment. In an embodiment, the VHT portion of the data unit also includes a VHT data portion that carries information to one or more users in the vicinity of the AP.

According to an embodiment, the preamble of a data unit includes a plurality of signal fields that carry information required at the receiver to properly identify and decode the data unit. In some embodiments, for example, two signal fields are included in the preamble, a first signal field included in a legacy portion of the preamble and modulated in a manner similar to the legacy portion of the data unit, and a second signal field included in a VHT portion of the preamble and modulated in a manner similar to the VHT data portion of the data unit. In one such embodiment, the second signal field is modulated similar to the VHT data portion of the data unit but using a lower coding rate and a smaller constellation size than the VHT data portion. Further, in some embodiments, bit allocation for the second signal field is the same regardless of the specific channel bandwidth that the data unit occupies. For example, in an embodiment, bit allocation is specified for the smallest possible bandwidth defined by the VHT protocol (e.g., 20 MHz bandwidth) and bit insertion and/or duplication is utilized to transmit the second signal field in a higher VHT bandwidth. Further, in an embodiment, the VHT data portion of a data unit includes multiple spatial data streams directed to a single user (SU) or multiple users (MU), while the second signal field is limited to a single data stream. In these embodiments, the single stream of the second signal field is mapped in some manner to the multiple space streams and/or multiple users corresponding to the data portion of the data unit.

FIG. 1 is a block diagram of an example embodiment of a wireless local area network (WLAN) 10 that utilizes various signal field modulation and mapping techniques described herein. An AP 14 includes a host processor 15 coupled to a network interface 16. The network interface 16 includes a medium access control (MAC) processing unit 18 and a physical layer (PHY) processing unit 20. The PHY processing unit 20 includes a plurality of transceivers 21, and the transceivers are coupled to a plurality of antennas 24. Although three transceivers 21 and three antennas 24 are illustrated in FIG. 1, the AP 14 can include different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers 21 and antennas 24 in other embodiments. In one embodiment, the MAC processing unit 18 and the PHY processing unit 20 are configured to operate according to a first communication protocol (e.g., the IEEE 802.11 ac Standard, now in the process of being standardized). The first communication protocol is also referred to herein as a very high throughput (VHT) protocol. In another embodiment, the MAC processing unit 18 and the PHY processing unit 20 are also configured to operate according to at least a second communication protocol (e.g., the IEEE 802.11n Standard, the IEEE 802.11a Standard, etc.).

The WLAN 10 includes a plurality of client stations 25. Although four client stations 25 are illustrated in FIG. 1, the WLAN 10 can include different numbers (e.g., 1, 2, 3, 5, 6, etc.) of client stations 25 in various scenarios and embodiments. At least one of the client stations 25 (e.g., client station 25-1) is configured to operate at least according to the first communication protocol.

The client station 25-1 includes a host processor 26 coupled to a network interface 27. The network interface 27 includes a MAC processing unit 28 and a PHY processing unit 29. The PHY processing unit 29 includes a plurality of transceivers 30, and the transceivers 30 are coupled to a plurality of antennas 34. Although three transceivers 30 and three antennas 34 are illustrated in FIG. 1, the client station 25-1 can include different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers 30 and antennas 34 in other embodiments.

In an embodiment, one or all of the client stations 25-2, 25-3 and 25-4, have a structure the same as or similar to the client station 25-1. In these embodiments, the client stations 25 structured the same as or similar to the client station 25-1 have the same or a different number of transceivers and antennas. For example, the client station 25-2 has only two transceivers and two antennas, according to an embodiment.

In various embodiments, the PHY processing unit 20 of the AP 14 is configured to generate data units conforming to the first communication protocol. The transceiver(s) 21 is/are configured to transmit the generated data units via the antenna(s) 24. Similarly, the transceiver(s) 24 is/are configured to receive the data units via the antenna(s) 24. The PHY processing unit 20 of the AP 14 is configured to process received data units conforming to the first communication protocol, according to an embodiment.

In various embodiments, the PHY processing unit 29 of the client device 25-1 is configured to generate data units conforming to the first communication protocol. The transceiver(s) 30 is/are configured to transmit the generated data units via the antenna(s) 34. Similarly, the transceiver(s) 30 is/are configured to receive data units via the antenna(s) 34. The PHY processing unit 29 of the client device 25-1 is configured to process received data units conforming to the first communication protocol, according to an embodiment.

FIG. 2 is a diagram of a data unit 250 that the AP 14 is configured to transmit to the client station 25-1, according to an embodiment. In an embodiment, the client station 25-1 is also configured to transmit the data unit 250 to the AP 14. The data unit 250 conforms to the VHT protocol and occupies an 80 MHz band. In other embodiments, data units similar to the data unit 250 occupy different bandwidths such as 20 MHz, 40 MHz, 120 MHz, 160 MHz, or any suitable bandwidth. Additionally, the band need not be contiguous in frequency, but may include two or more smaller bands separated in frequency. For example, according to an embodiment, the data unit 250 occupies a 160 MHz band composed of two non-contiguous 80 MHz bands separated in frequency by some suitable minimum bandwidth, in some scenarios such as when conditions and devices support a 160 MHz channel. The data unit 250 includes a preamble having four legacy short training fields (L-STFs) 252, four legacy long training fields (L-LTFs) 254, four legacy signal fields (L-SIGs) 256, four first very high throughput signal fields (VHT-SIGAs) 258 a very high throughput short training field (VHT-STF) 262, N very high throughput long training fields (VHT-LTFs) 264, where N is an integer, and a second very high throughput signal field (VHT-SIGB) 268. The data unit 250 also includes a data portion 272. The L-STFs 252, the L-LTFs 254, and the L-SIGs 256 form a legacy portion. The VHT-STF 262, the VHT-SIGAs 258, the VHT-LTFs 264, the VHT-SIGB 268, and the data portion 266 form a very high throughput (VHT) portion.

In the embodiment of FIG. 2, each of the L-STFs 252, each of the L-LTFs 254, each of the L-SIGs 256, and each of the VHT-SIGAs 258, occupy a 20 MHz band. In the present disclosure, several example data units, including the data unit 250, having an 80 MHz contiguous bandwidth are described for the purposes of illustrating embodiments of frame formats, but these frame format embodiments and other embodiments are applicable to other suitable bandwidths (including noncontiguous bandwidths). For instance, although the preamble of FIG. 2 includes four of each of the L-STFs 252, the L-LTFs 254, the L-SIGs 256, and the VHT-SIGAs 258, in other embodiments in which the orthogonal frequency division multiplex (OFDM) data unit occupies a cumulative bandwidth other than 80 MHz, such as 20 MHz, 40 MHz, 120 MHz, 160 MHz, etc., a different suitable number of the L-STFs 252, the L-LTFs 254, the L-SIGs 256, and the VHT-SIGAs 258 is utilized accordingly (e.g., one of each of the L-STFs 252, the L-LTFs 254, the L-SIGs 256, and the VHT-SIGAs 258, for an OFDM data unit occupying 20 MHz, two of each of the fields for a 40 MHz bandwidth OFDM data unit, six of each of the fields for a 120 MHz bandwidth OFDM data unit, and eight of each of the fields for a 160 MHz bandwidth OFDM data unit). Also in a 160 MHz bandwidth OFDM data unit, for example, the band is not contiguous in frequency, in some embodiments and situations. Thus, for example, the L-STFs 252, the L-LTFs 254, the L-SIGs 256, and the VHT-SIGAs 258 occupy two or more bands that are separated from each other in frequency, and adjacent bands are separated in frequency by at least one MHz, at least five MHz, at least 10 MHz, at least 20 MHz, for example, in some embodiments. In the embodiment of FIG. 2, each of the VHT-STF 262, the VHT-LTFs 264, the VHT-SIGB 268, and the data portion 266 occupy an 80 MHz band. If the data unit conforming to the first communication protocol is an OFDM data unit that occupies a cumulative bandwidth such as 20 MHz, 40 MHz, 120 MHz, or 160 MHz OFDM, the VHT-STF, VHT-LTFs, VHT-SIGB and VHT data portion occupy the corresponding whole bandwidth of the data unit, according to an embodiment.

Further, according to the embodiment of FIG. 2 in which the device generating the data unit 250 includes multiple antennas and is capable of transmit beamforming or beamsteering, the VHT-SIGA 258 is included within an unsteered (or “omnidirectional” or “pseudo-omnidirectional”; the terms “unsteered” and “omnidirectional” as used herein are intended to also encompass the term “pseudo-omnidirectional”) portion of the data unit 250 and contains PHY information that is common to each of the client stations 25 in FIG. 1. On the other hand, the VHT-SIGB 268 is contained in a “steered” portion. In an embodiment in which the data unit 250 is a multi-user transmission (e.g., the data unit 250 includes independent data streams for corresponding different receive devices), the steered portion includes different data for different clients 25 that are simultaneously transmitted, via the antennas 24 in FIG. 1, over different spatial channels to carry different (or “user-specific”) content to each of the client stations 25. Accordingly, in these embodiments the VHT-SIGAs 258 carry information common to all users, while the VHT-SIGB 268 includes user-specific information. On the other hand, in an embodiment in which the data unit 250 is a single-user transmission; the steered portion includes data for a particular client 25 that are transmitted and beamsteered, via the antennas 24, to the client station 25.

According to an embodiment, each the VHT-SIGAs 258 comprises two OFDM symbols that are modulated in a manner similar to the legacy L-SIG fields 256. On the other hand, the VHT-SIGB field 268 comprises a single OFDM symbol that is modulated in a manner similar to the VHT data portion 272, according to some embodiments and/or scenarios described below.

FIG. 3 is a block diagram of an example PHY processing unit 300 configured to generate an OFDM symbol corresponding to the VHT-SIGB 268 of the data unit 250 (FIG. 2), according to an embodiment. Referring to FIG. 1, the AP 14 and the client station 25-1, in one embodiment, each include a PHY processing unit such as the PHY processing unit 300.

According to an embodiment, the PHY unit 300 includes a forward error correction (FEC) encoder 302 that generally encodes an input data stream to generate a corresponding encoded stream. In one embodiment, the FEC encoder utilizes binary convolutional coding (BCC) with the coding rate of ½. In other embodiments, the FEC encoder utilizes other suitable coding types and/or other suitable coding rates. The FEC encoder 302 is coupled to a frequency interleaver 304 that interleaves bits of an encoded stream (i.e., changes the order of the bits) to prevent long sequences of adjacent noisy bits from entering a decoder at the receiver.

A constellation mapper 306 maps an interleaved sequence of bits to constellation points corresponding to different subcarriers of an OFDM symbol. More specifically, the constellation mapper 306 translates every log₂(M) into one of M constellation points. In one embodiment, the constellation mapper 306 operates according to a binary phase shift keying (BPSK) modulation scheme. In other embodiments, other suitable modulation schemes are utilized. The constellation mapper 306 is coupled to a tone duplication and insertion unit 308 that implements various duplication and insertion techniques described below in various embodiments and/or scenarios.

The output of the tone duplication and insertion unit 308 is presented to a stream mapper unit 312, according to an embodiment. In an embodiment, the stream mapper 312 spreads the constellation points to a greater number of space-time streams. A pilot generator unit 310 generates pilot tones to be used, for example, for frequency offset estimation at the receiver, and insets the pilot tones into the symbol OFDM tones at the space-time outputs of the stream mapper 312. A plurality of cyclic shift diversity (CSD) units 314 insert cyclic shifts into all but one of the space-time streams to prevent unintentional beamforming.

A spatial mapping unit 316 maps the space-time streams to transmit chains corresponding to one or more available transmit antennas. In various embodiments, spatial mapping includes one or more of: 1) direct mapping, in which constellation points from each space-time stream are mapped directly onto transmit chains (i.e., one-to-one mapping); 2) spatial expansion, in which vectors of constellation point from all space-time, streams are expanded via matrix multiplication to produce inputs to the transmit chains; and 3) beamforming, in which each vector of constellation points from all of the space-time streams is multiplied by a matrix of steering vectors to produce inputs to the transmit chains.

In one embodiment, the spatial mapping unit 316 applies a steering matrix Q (e.g., multiplies an N_(STS)×1 signal vector s by Q, i.e., Qs), where Q has a size of (N_(TX)×N_(STS)), where N_(TX) is the number of transmit chains and N_(STS) is the number of space-time streams. When beamforming is utilized, the matrix Q is generated based on the multiple input multiple output (MIMO) channel between the transmitter and the receiver. In one embodiment, N_(TX) has a maximum value of 8. In another embodiment, N_(TX) has a maximum value of 16. In other embodiments, N_(TX) has a different maximum value such as 4, 32, 64, etc.

Each output of the spatial mapping unit 316 corresponds to a transmit chain, and each output of the spatial mapping unit 316 is operated on by an inverse discrete Fourier transform (IDFT) unit 318 that converts a block of constellation points to a time-domain signal. In an embodiment, the IDFT unit 318 is configured to implement an inverse fast Fourier transform (IFFT) algorithm. Each time-domain signal is provided to a transmit antenna for transmission.

The number of sub-carriers (or tones) in an OFDM symbol generally depends on the bandwidth (BW) of the channel being utilized, according to an embodiment. For example, an OFDM symbol for a 20 MHz channel corresponds to a size 64 IDFT and includes 64 tones, whereas an OFDM symbol for a 40 MHz channel corresponds to a size 128 IDFT and includes 128 tones, according to an embodiment. In an embodiment, the tones in an OFDM symbol include guard tones for filter ramp up and ramp down, DC tones for mitigating radio frequency interference, and pilot tones for frequency offset estimation. The remaining tones can be used to transmit data or information bits (“data tones”), according to an embodiment. General transmitter flow of an example PHY processing unit configured to generate data units conforming to the first communication protocol as well as various example transmission channels and tone mappings that are utilized in the data units corresponding to some embodiments of the present disclosure are described in U.S. patent application Ser. No. 12/846,681, entitled “Methods and Apparatus for WLAN Transmission”, filed on Jul. 29, 2010, which is hereby incorporated by reference herein in its entirety.

Referring now to FIG. 2, the bit allocation for the VHT-SIGB field 268 of the data unit 250 is the same regardless of the channel bandwidth occupied by the particular data unit being generated, according to an embodiment. Also, in some embodiments, the same number of guard tones, DC tones, and pilot tones are used in an OFDM symbol generated for the VHT-SIGB 268 as in a symbol generated for the data portion of the data unit 250. In one such embodiment, the guard tones, the DC tones, and the pilot tones are the same frequency tones within an OFDM symbol generated for the VHT-SIGB field 268 as in an OFDM symbol generated for the data portion 272.

In an embodiment, VHT-SIGB field 268 bit allocation corresponds to a 20 MHz OFDM symbol with the corresponding number of data tones, and the same bit allocation is utilized for data units corresponding to larger bandwidths (e.g., 40 MHz, 80 MHz, etc.). In one such embodiment, 26 bits are allocated for the VHT-SIGB field, with 20 bits allocated for information bits and 6 bits allocated for tail bits, for example. In an embodiment in which VHT-SIGB field 268 is encoded with a BCC encoder at ½ coding rate, the 26 bits are encoded into 52 data bits corresponding to the 52 data tones available for a 20 MHz channel. In other embodiments, other suitable bit allocations and other suitable coding and modulation schemes are used for the VHT-SIGB field 268. In various embodiments and/or scenarios in which the same number of bits is allocated for larger bandwidth channels with a corresponding larger number of data tones, tone duplication and insertion techniques described herein are utilized to fill the remaining available data tones.

FIG. 4 is a diagram of an OFDM symbol 400 generated for a VHT-SIGB field (such as VHT-SIGB field 268 of FIG. 2) of a data unit for a 40 MHz channel, according to an embodiment. The OFDM symbol 400 corresponds to a size 128 IDFT and includes 128 tones. The 128 tone slots are indexed from −64 to +63, in an embodiment. The 128 tones include guard tones, a direct current (DC) tones, data tones, and pilot tones. The six lowest frequency tones and the five highest frequency tones are guard tones. The three tones indexed from −1 to +1 are DC tones. The OFDM symbol 400 also includes 6 pilot tones and 108 data tones, according to an embodiment. As illustrated in FIG. 4, the 108 data tones include 52 tones corresponding to the VHT-SIGB bits with 2 inserted tones, and the resulting 54 tones are duplicated once in order to fill the remaining tones of the OFDM symbol. In the OFDM symbol 400, the two inserted tones occupy the lowest data/pilot frequency tone slots in the lower channel sideband and the two lowest data/pilot frequency tone slots in the upper channel sideband.

FIG. 5 is a diagram of another example OFDM symbol 500 generated for a VHT-SIGB field (such as VHT-SIGB field 268 of FIG. 2) of a data unit for a 40 MHz channel, according to another embodiment. The OFDM symbol 500 is similar to the OFDM symbol 400 except that the insertion tones in the OFDM symbol 500 occupy the two lowest data/pilot frequency tone slots in the lower channel sideband and the two highest data/pilot frequency tone slots in the upper channel sideband.

In other embodiments, the two insertion tones occupy any other suitable data/pilot frequency tone slots in the OFDM symbol 400 or the OFDM symbol 500.

FIG. 6 is a diagram of an OFDM symbol 600 generated for a VHT-SIGB field (such as VHT-SIGB field 268 of FIG. 2) of a data unit for an 80 MHz channel, according to an embodiment. The OFDM symbol 600 corresponds to a size 256 IDFT and includes 256 tones. The 256 tone slots are indexed from −128 to +127, in an embodiment. The 256 tones include guard tones, DC tones, data tones, and pilot tones. The six lowest frequency tones and the five highest frequency tones are guard tones. The three tones indexed from −1 to +1 are DC tones. The OFDM symbol 350 also includes 8 pilot tones and 234 data tones. The 234 data tones include 52 tones corresponding to the VHT-SIGB information bits, 52 tones that are duplicates of the VHT-SIGB information bits and 13 inserted tones, and the resulting 117 tones duplicated once. In the OFDM symbol 600, the thirteen inserted tones occupy the lowest frequency pilot/data tone slots in the lower channel sideband and the lowest frequency pilot/data tone slots in the upper channel sideband.

FIG. 7 is a diagram of another OFDM symbol 700 (such as VHT-SIGB field 268 of FIG. 2) generated for a VHT-SIGB field of a data unit for an 80 MHz channel, according to another embodiment. The OFDM symbol 700 is similar to the OFDM symbol 600 except that the insertion tones in the OFDM symbol 700 occupy the thirteen lowest frequency data/pilot tone slots in the lower channel sideband and the highest frequency data/pilot tone slots in the upper channel sideband.

In other embodiments, the thirteen insertion tones occupy other suitable data/pilot tone slots in the OFDM symbol 600 or the OFDM symbol 700.

According to an embodiment or a situation, the insertion tones in symbol 400, the insertion tones in the symbol 500, the insertion tones in the symbol 600, and/or the insertion tones in the symbol 700 carry values of some of the VHT-SIGB information bits and/or VHT-SIGA information bits. Similarly, in some other embodiments and/or situations, the insertion tones in symbol 400, the insertion tones in the symbol 500, the insertion tones in the symbol 600, and/or the insertion tones in the symbol 700 carry values of some of the LSIG information bits. Alternatively, in other embodiments and/or situations, the insertion tones in symbol 400, the insertion tones in the symbol 500, the insertion tones in the symbol 600, and/or the insertion tones in the symbol 700 are null (0) tones. These embodiments have an advantage of using no extra transmit power for transmitting the insertion tones (i.e., all of the transmit power is used for the VHT-SIGB information and tail bits). In other embodiment and/or scenarios, the insertion tones in symbol 400, the insertion tones in the symbol 500, the insertion tones in the symbol 600, the insertion tones in the symbol 700 tones are modulated with any other suitable values.

In other embodiments and/or scenarios, the insertion tones in symbol 400, the insertion tones in the symbol 500, the insertion tones in the symbol 600, and/or the insertion tones in the symbol 700 are modulated with any other suitable values.

In an embodiment, the client station 25-1 in FIG. 1 discards the inserted tones in a VHT-SIGB field of a received data unit during the decoding and demodulation process. Alternatively, if the inserted tones are of values corresponding to some information bits of a signal field (e.g., VHT-SIGA, VHT-SIGB, L-SIG), the receiver utilizes the extra diversity provided thereby during the decoding and demodulating process rather than simply discarding the inserted tones, according to an embodiment.

Referring to FIG. 2, in embodiments in which the data portion 272 includes multiple spatial streams, the VHT-SIGB field 268 is mapped to the multiple streams accordingly. In some such embodiments, the VHT-STF fields 264 that contain training sequences corresponding to the multiple spatial streams are mapped to multiple spatial streams via a matrix P. In some embodiments and/or scenarios, the same matrix P is used to map a single data stream in the VHT-SIGB field 268 to multiple data streams corresponding to multiple spatial streams in the VHT-data portion 272. More specifically, in an embodiment, the VHT-LTF training fields 264 are mapped to the corresponding spatial streams according to:

VHTLTF^((k))=[L₁, L₂, . . . L_(N) _(LTF) ]=Q^((k))D^((k))[P_(*1), P_(*2), . . . P_(*N) _(LTF) ]s^((k))  (Equation 1

where Q^((k)) corresponds to spatial mapping of the k^(th) tone of a VHT-LTF training field, D^((k)) corresponds to a CSD phase shift for the k^(th) tone, P_(*1), P_(*2), . . . P_(*NLTF) are columns of the mapping matrix P, and S^((k)) is the k^(th) tone of a VHT-LTF training symbol.

Referring again to FIG. 2, according to an embodiment, the VHT-SIGB field 268 is mapped to multiple spatial streams of the data unit 250 using one of the columns P_(*1), . . . , P_(*NLTF) of Equation 1. For example, in an embodiment, the first column of the P matrix is used to map the VHT-SIGB field 268:

VHTSIGB^((k))=Q^((k))D^((k))P_(*1)s_(VHTSIGB) ^((k))  Equation 2

where S_(VHTSIGB) _(—) _(U1) ^((k)) is the k^(th) tone of the VHT-SIGB symbol. In other embodiments and/or scenarios, a different column of the P matrix is used to map the VHT-SIGB field 268.

In some embodiments, the data unit 250 is a multiuser (MU) data unit, i.e., the data unit 250 includes user-specific information for more than one user (e.g., more than one of the client stations 25 in FIG. 1). For example, the data unit 250 includes use-specific information for two users (i.e., the data unit 250 is a “two-user” data unit), according to an embodiment. The data unit 250 includes data for different numbers of users (e.g., 3 users, 4 users, 5 users, etc.) in other embodiments and/or scenarios. In some such embodiments, the number of VHT-LTF fields 264 is directly related to the sum of spatial streams for all intended recipients of the data unit (users), and a single “giant” mapping matrix P is used to jointly map the training information tones for all users and all spatial streams. For example, if the data unit 250 is a two-user data unit, the VHT-LTF fields 268 are mapped, in an embodiment, according to:

$\begin{matrix} \begin{matrix} {{VHTLTF}^{(k)} = \left\lbrack {L_{1},L_{2},{\ldots \mspace{14mu} L_{N_{LTF}}}} \right\rbrack} \\ {= {\left\lbrack {Q_{U\; 1}^{(k)},Q_{U\; 2}^{(k)}} \right\rbrack \begin{bmatrix} D_{U\; 1}^{(k)} & 0 \\ 0 & D_{U\; 2}^{(k)} \end{bmatrix}}} \\ {{\begin{bmatrix} P_{{({U\; 1})}\_^{*}1} & P_{{({U\; 1})}\_^{*}2} & \ldots & P_{{({U\; 1})}\_^{*}N_{LTF}} \\ P_{{({U\; 2})}\_^{*}1} & P_{{({U\; 2})}\_^{*}2} & \ldots & P_{{({U\; 2})}\_^{*}N_{LTF}} \end{bmatrix}s^{(k)}}} \end{matrix} & {{Equation}\mspace{14mu} 3} \end{matrix}$

where Q_(U1) ^((k)) corresponds to spatial mapping of the k^(th) tone of a VHT-LTF training field for user 1, Q_(U2) ^((k)) corresponds to spatial mapping of the k^(th) tone of a VHT-LTF training field for user 2, D_(U1) ^((k)) corresponds to a cyclic shift diversity (CSD) phase shift for the k^(th) tone for user 1, D_(U2) ^((k)) corresponds to a cyclic shift diversity (CSD) phase shift for the k^(th) tone for user 2, P_(U1) _(—) _(*1), . . . , P_((U1)) _(—) _(NLTF) are columns of the mapping matrix P for user 1, P_((U2)) _(—) _(*1), . . . , P_((U2)) _(—) _(*NLTF) are columns of the mapping matrix P for user 2, and S^((k)) is the k^(th) tone of a VHT-LTF training symbol.

With reference to FIG. 2, according to an embodiment in which the data unit 250 is a two-user data unit, the VHT-SIGB field 268 is, therefore, steered to the two users (assuming that each user does not see interference from the other user). In this case, the single stream of the VHT-SIGB filed 268 is mapped to multiple spatial streams and the multiple users using any column P_((U1)) _(—) _(*1), . . . , P_((U1)) _(—) _(*NLTF) or P_((U2)) _(—) _(*1), . . . , P_((U1)) _(—) _(*NLTF) of Equation 3. For example, in an embodiment, the first column of the joint P matrix is used to map the VHT-SIGB field 268 for user 1 according to:

VHTSIGB_(U1) ^((k))=Q_(U1) ^((k))D_(U1) ^((k))P_((U2)) _(—) _(*1)s_(VHTSIGB) _(—) _(U1) ^((k))  Equation 4

where S_(VHTSIGB) _(—) _(U1) ^((k)) is the VHT-SIGB symbol k^(th) tone for user 1. In other embodiments, other columns of the joint P matrix are used to steer the VHT-SIGB field 268 to the intended user via the multiple data streams.

FIG. 8 is a flow diagram of an example method 800 for generating and transmitting a PHY data unit having a signal field, such as a VHT-SIGB or another suitable field, according to an embodiment. The method 800 is implemented at least partially by a PHY processing unit such as the PHY processing unit 20 (FIG. 1), the PHY processing unit 29 (FIG. 1), and/or the PHY processing unit 300 (FIG. 3), and FIG. 8 will be described with reference to FIG. 3 for ease of explanation. In other embodiments, however, another suitable PHY processing unit and/or network interface implements the method 800.

At block 804, a signal field of a preamble of a PHY data unit is generated. In an embodiment, the VHT-SIGB field is generated. In another embodiment, another suitable signal field is generated.

At block 808, the signal field generated at block 804 is mapped to a first plurality of data subcarriers corresponding to a first frequency portion of an OFDM symbol. For example, the BPSK constellation mapping block 306 maps the signal field to a first plurality of data subcarriers corresponding to a first frequency portion of an OFDM symbol. In another embodiment, another suitable processing block of a network interface implements block 808.

At block 812, a set of data subcarriers in the first plurality of data subcarriers are set to predetermined values. For example, in an embodiment, at least some of the subcarriers in the set of subcarriers are set to “+1” value or some other suitable value. As another example, in an embodiment, at least some of the subcarriers in the set of subcarriers are set to “−1” value or some other suitable value. As another example, in an embodiment, at least some of the subcarriers in the set of subcarriers are set to a null value. In an embodiment, the block 812 is implemented by the tone duplications and insertions block 308 in FIG. 3. In another embodiment, another suitable processing block of a network interface implements block 812.

At block 816, the signal field generated at block 804 is mapped to a second plurality of data subcarriers corresponding to a second frequency portion of the OFDM symbol. For example, the tone duplications and insertions block 308 in FIG. 3 maps the signal field to a second plurality of data subcarriers corresponding to the second frequency portion of the OFDM symbol. In another embodiment, another suitable processing block of a network interface implements block 816.

At block 820, a set of data subcarriers in the second plurality of data subcarriers are set to predetermined values. For example, in an embodiment, at least some of the subcarriers in the set of subcarriers are set to “+1” value or some other suitable value. As another example, in an embodiment, at least some of the subcarriers in the set of subcarriers are set to “−1” value or some other suitable value. As another example, in an embodiment, at least some of the subcarriers in the set of subcarriers are set to a null value. In an embodiment, the block 820 is implemented by the tone duplications and insertions block 308 in FIG. 3. In another embodiment, another suitable processing block of a network interface implements block 820.

At block 824, guard tones, DC tones, and/or pilot tones in the first frequency portion and the second frequency portion are set. In an embodiment, the block 824 is implemented at least partially by the VHT pilots generation block 310. In another embodiment, another suitable processing block of a network interface implements block 824.

At block 828, the PHY data unit is transmitted. For example, in an embodiment, a PHY processing unit that implements the method 800 at least partially causes the PHY data unit to be transmitted.

FIG. 9 is a flow diagram of another example method 900 for generating and transmitting a PHY data unit having a signal field, such as a VHT-SIGB or another suitable field, according to an embodiment. The method 900 is implemented at least partially by a PHY processing unit such as the PHY processing unit 20 (FIG. 1), the PHY processing unit 29 (FIG. 1), and/or the PHY processing unit 300 (FIG. 3), and FIG. 9 will be described with reference to FIG. 3 for ease of explanation. In other embodiments, however, another suitable PHY processing unit and/or network interface implements the method 900.

At block 904, a plurality of training fields are generated. For example, in an embodiment, a plurality of VHT-LTF fields are generated, in an embodiment. At block 908, the training fields are mapped to signal streams using a mapping matrix. In an embodiment, the mapping matrix comprises the matrix P discussed above. In other embodiments, other suitable mapping matrices are utilized. In an embodiment, the block 908 is implemented by the mapping block 312. In other embodiments, however, another suitable block of a PHY processing unit and/or a network interface implements block 908.

At block 912, a signal field of a preamble of a PHY data unit is generated. In an embodiment, the VHT-SIGB field is generated. In another embodiment, another suitable signal field is generated. At block 916, the signal field is mapped to a plurality of signal streams using a column of the mapping matrix utilized at block 908. In an embodiment, a column of the matrix P discussed above is utilized. In other embodiments, a column of another suitable mapping matrix is utilized. In an embodiment, the first column of the matrix P is utilized. In other embodiments, a column other than the first column of the matrix P is utilized.

At block 920, the signal streams are mapped to spatial streams. In an embodiment, the signal streams are mapped to spatial streams using the matrix Q discussed above. In other embodiments, other suitable matrices are utilized. In an embodiment, the block 920 is implemented by the spatial mapping block 316. In other embodiments, however, another suitable block of a PHY processing unit and/or a network interface implements block 920.

At block 924, the PHY data unit is transmitted. For example, in an embodiment, a PHY processing unit that implements the method 900 at least partially causes the PHY data unit to be transmitted. Block 924 includes transmitting (or causing to be transmitted) at least i) the plurality of training fields, and ii) the signal field, via the plurality of spatial streams.

FIG. 10 is a flow diagram of another example method 950 for generating and transmitting a multi-user PHY data unit having a signal field, such as a VHT-SIGB or another suitable field, according to an embodiment. The method 950 is implemented at least partially by a PHY processing unit such as the PHY processing unit 20 (FIG. 1), the PHY processing unit 29 (FIG. 1), and/or the PHY processing unit 300 (FIG. 3), and FIG. 10 will be described with reference to FIG. 3 for ease of explanation. In other embodiments, however, another suitable PHY processing unit and/or network interface implements the method 950.

At block 954, a plurality of training fields are generated for a multi-user PHY data unit. For example, in an embodiment, a plurality of VHT-LTF fields are generated. At block 958, the training fields are mapped to signal streams using a mapping matrix. In an embodiment, the mapping matrix comprises the giant matrix P discussed above. In other embodiments, other suitable mapping matrices are utilized. In an embodiment, the block 958 is implemented by the mapping block 312. In other embodiments, however, another suitable block of a PHY processing unit and/or a network interface implements block 958.

At block 962, a first signal field of a preamble of the multi-user PHY data unit is generated, wherein the first signal field corresponds to a first client device. In an embodiment, the VHT-SIGB field is generated. In another embodiment, another suitable signal field is generated. At block 966, the first signal field is mapped to a plurality of signal streams using a portion of a column of the mapping matrix utilized at block 958, wherein the portion corresponds to the first client device. In an embodiment, a portion of a column of the giant matrix P discussed above is utilized, wherein the portion corresponds to the first client device. In other embodiments, a portion of a column of another suitable mapping matrix is utilized. In an embodiment, a portion of the first column of the giant matrix P is utilized. In other embodiments, a portion of a column other than the first column of the giant matrix P is utilized.

At block 970, a second signal field of a preamble of the multi-user PHY data unit is generated, wherein the second signal field corresponds to a second client device. In an embodiment, the VHT-SIGB field is generated. In another embodiment, another suitable signal field is generated. At block 974, the second signal field is mapped to a plurality of signal streams using a portion of the column of the mapping matrix utilized at block 958, wherein the portion corresponds to the second client device. In an embodiment, a portion of a column of the giant matrix P discussed above is utilized, wherein the portion corresponds to the second client device. In other embodiments, a portion of a column of another suitable mapping matrix is utilized. In an embodiment, a portion of the first column of the giant matrix P is utilized. In other embodiments, a portion of a column other than the first column of the giant matrix P is utilized. In an embodiment, the same column is utilized in blocks 966 and 974.

At block 978, the signal streams are mapped to spatial streams. In an embodiment, the signal streams are mapped to spatial streams using a matrix Q as discussed above. In other embodiments, other suitable matrices are utilized. In an embodiment, the block 978 is implemented by the spatial mapping block 316. In other embodiments, however, another suitable block of a PHY processing unit and/or a network interface implements block 978.

At block 982, the multi-user PHY data unit is transmitted. For example, in an embodiment, a PHY processing unit that implements the method 950 at least partially causes the PHY data unit to be transmitted. Block 982 includes transmitting (or causing to be transmitted) at least i) the plurality of training fields, ii) the first signal field, and iii) the second signal field via the plurality of spatial streams.

At least some of the various blocks, operations, and techniques described above may be implemented utilizing hardware, a processor executing firmware instructions, a processor executing software instructions, or any combination thereof. When implemented utilizing a processor executing software or firmware instructions, the software or firmware instructions may be stored in any computer readable memory such as on a magnetic disk, an optical disk, or other storage medium, in a RAM or ROM or flash memory, processor, hard disk drive, optical disk drive, tape drive, etc. Likewise, the software or firmware instructions may be delivered to a user or a system via any known or desired delivery method including, for example, on a computer readable disk or other transportable computer storage mechanism or via communication media. Communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, radio frequency, infrared and other wireless media. Thus, the software or firmware instructions may be delivered to a user or a system via a communication channel such as a telephone line, a DSL line, a cable television line, a fiber optics line, a wireless communication channel, the Internet, etc. (which are viewed as being the same as or interchangeable with providing such software via a transportable storage medium). The software or firmware instructions may include machine readable instructions that, when executed by the processor, cause the processor to perform various acts.

When implemented in hardware, the hardware may comprise one or more of discrete components, an integrated circuit, an application-specific integrated circuit (ASIC), etc.

While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, changes, additions and/or deletions may be made to the disclosed embodiments without departing from the scope of the invention. 

1. A method, comprising: generating a plurality of training fields of a preamble of a physical layer data unit; mapping training fields of the plurality of training fields to respective signal streams of a plurality of streams using a mapping matrix; generating a signal field of the preamble of the data unit; mapping the signal field to the plurality of signal streams using a column of the mapping matrix; mapping the plurality of signal streams to a plurality of spatial streams; and transmitting at least i) the plurality of training fields, and ii) the signal field via the plurality of spatial streams.
 2. A method according to claim 1, wherein mapping the signal field to the plurality of signal streams using a column of the mapping matrix comprises using the left-most column of the mapping matrix.
 3. A method according to claim 1, wherein the signal field corresponds to a single orthogonal frequency division multiplex (OFDM) symbol, and wherein the method further comprises: encoding the signal field according to a ½ rate binary convolutional code; and modulating the signal field using binary phase shift keying.
 4. An apparatus, comprising: a network interface, including a physical layer processing unit configured to: generate a plurality of training fields of a preamble of a physical layer data unit, map training fields of the plurality of training fields to respective signal streams of a plurality of streams using a mapping matrix, generate a signal field of the preamble of the data unit, map the signal field to the plurality of signal streams using a column of the mapping matrix, map the plurality of signal streams to a plurality of spatial streams, and cause at least i) the plurality of training fields, and ii) the signal field to be transmitted via the plurality of spatial streams.
 5. An apparatus according to claim 4, wherein mapping the signal field to the plurality of signal streams using a column of the mapping matrix comprises using the left-most column of the mapping matrix.
 6. An apparatus according to claim 4, wherein the physical layer processing unit comprises: an encoder configured to encode the signal field using a binary convolutional code; a constellation mapping unit configured to map the signal field to subcarriers of an orthogonal frequency division multiplex (OFDM) symbol; a signal stream mapper configured to i) map the training fields of the plurality of training fields to respective signal streams of the plurality of streams using the mapping matrix, and ii) map the signal field to the plurality of signal streams using the column of the mapping matrix; and a spatial stream mapper configured to map the plurality of signal streams to the plurality of spatial streams.
 7. A method, comprising: generating a plurality of training fields of a preamble of a multi-user physical layer data unit; mapping training fields of the plurality of training fields to respective signal streams of a plurality of streams using a mapping matrix, wherein different portions of each column of the mapping matrix correspond to different client devices including a first client device and a second client device; generating a first signal field of the preamble of the data unit corresponding to the first client device; mapping the first signal field to the plurality of signal streams corresponding to the first client device using a portion of a column of the mapping matrix corresponding to the first client device; generating a second signal field of the preamble of the data unit corresponding to the second client device; mapping the second signal field to the plurality of signal streams corresponding to the second client device using another portion of the column of the mapping matrix corresponding to the second client device; mapping the plurality of signal streams to a plurality of spatial streams; and transmitting at least i) the plurality of training fields, ii) the first signal field, and iii) the second signal field via the plurality of spatial streams.
 8. A method according to claim 7, wherein the portion of the column of the mapping matrix corresponding to the first client device and used to map the first signal field to the plurality of signal streams corresponds to a portion of the left-most column of the mapping matrix, and wherein the portion of the column of the mapping matrix corresponding to the second client device and used to map the second signal field to the plurality of signal streams corresponds to another portion of the left-most column of the mapping matrix.
 9. A method according to claim 7, wherein each of the first signal field and the second signal field corresponds to a single orthogonal frequency division multiplex (OFDM) symbol, and wherein the method further comprises: encoding each of the first signal field and the second signal field according to a ½ rate binary convolutional code; and modulating each of the first signal field and the second signal field using binary phase shift keying.
 10. An apparatus, comprising: a network interface, including a physical layer processing unit configured to: generate a plurality of training fields of a preamble of a multi-user physical layer data unit, map training fields of the plurality of training fields to respective signal streams of a plurality of streams using a mapping matrix, wherein different portions of each column of the mapping matrix correspond to different client devices including a first client device and a second client device, generate a first signal field of the preamble of the data unit, map the first signal field to the plurality of signal streams using a portion of a column of the mapping matrix corresponding to the first client device, generate a second signal field of the preamble of the data unit corresponding to the second client device, map the second signal field to the plurality of signal streams corresponding to the second client device using another portion of the column of the mapping matrix corresponding to the second client device, map the plurality of signal streams to a plurality of spatial streams, and cause at least i) the plurality of training fields, ii) the first signal field, and iii) the second signal field to be transmitted via the plurality of spatial streams.
 11. An apparatus according to claim 10, wherein the portion of the column of the mapping matrix corresponding to the first client device and used to map the first signal field to the plurality of signal streams corresponds to a portion of the left-most column of the mapping matrix, and wherein the portion of the column of the mapping matrix corresponding to the second client device and used to map the second signal field to the plurality of signal streams corresponds to another portion of the left-most column of the mapping matrix.
 12. An apparatus according to claim 10, wherein the physical layer processing unit comprises: an encoder configured to encode each of the first signal field and the second signal field using a binary convolutional code; a constellation mapping unit configured to map each of the first signal field and the second signal field to subcarriers of a respective orthogonal frequency division multiplex (OFDM) symbol; a signal stream mapper configured to i) map the training fields of the plurality of training fields to respective signal streams of the plurality of streams using the mapping matrix, and ii) map the first signal field and the second signal field to the plurality of signal streams using the column of the mapping matrix; and a spatial stream mapper configured to map the plurality of signal streams to the plurality of spatial streams.
 13. A method, comprising: generating a signal field of a preamble of a physical layer data unit; mapping, the signal field to a first plurality of data subcarriers corresponding to a first frequency portion of an orthogonal frequency division multiplex (OFDM) symbol; setting a subset of data subcarriers in the first plurality of data subcarriers to one or more predetermined values; mapping the signal field to a second plurality of subcarriers corresponding to a second frequency portion of the OFDM symbol; setting a subset of subcarriers in the second plurality of subcarriers to one or more predetermined values; setting guard tones, direct current (DC) tones, and pilot tones in the first frequency portion and the second frequency portion; and transmitting the physical layer data unit that includes the OFDM symbol in the preamble of the physical layer data unit.
 14. A method according to claim 13, wherein setting the subset of data subcarriers in the first plurality of data subcarriers to one or more predetermined values comprises setting at least one data subcarrier in the subset of data subcarriers in the first plurality of data subcarriers to a null value; and wherein setting the subset of data subcarriers in the second plurality of data subcarriers to one or more predetermined values comprises setting at least one data subcarrier in the subset of data subcarriers in the second plurality of data subcarriers to the null value.
 15. A method according to claim 13, wherein setting the subset of data subcarriers in the first plurality of data subcarriers to one or more predetermined values comprises setting at least one data subcarrier in the subset of data subcarriers in the first plurality of data subcarriers to a non-zero value; and wherein setting the subset of data subcarriers in the second plurality of data subcarriers to one or more predetermined values comprises setting at least one data subcarrier in the subset of data subcarriers in the second plurality of data subcarriers to the non-zero value.
 16. A method according to claim 13, further comprising: mapping the signal field to a third plurality of data subcarriers corresponding to a third frequency portion of the OFDM symbol; setting a subset of data subcarriers in the third plurality of data subcarriers to one or more predetermined values; mapping the signal field to a fourth plurality of subcarriers corresponding to a fourth frequency portion of the OFDM symbol; setting a subset of subcarriers in the fourth plurality of subcarriers to one or more predetermined values; and setting guard tones and pilot tones in the third frequency portion and the fourth frequency portion.
 17. An apparatus, comprising: a network interface, including a physical layer processing unit configured to: generate a signal field of a preamble of a physical layer data unit, map the signal field to a first plurality of data subcarriers corresponding to a first frequency portion of an orthogonal frequency division multiplex (OFDM) symbol, set a subset of data subcarriers in the first plurality of data subcarriers to one or more predetermined values, map the signal field to a second plurality of subcarriers corresponding to a second frequency portion of the OFDM symbol, set a subset of subcarriers in the second plurality of subcarriers to one or more predetermined values, set guard tones, direct current (DC) tones, and pilot tones in the first frequency portion and second frequency portion, and cause the physical layer data unit that includes the OFDM symbol in the preamble of the physical layer data unit to be transmitted.
 18. An apparatus according to claim 17, wherein the physical layer processing unit is configured to: set at least one data subcarrier in the subset of data subcarriers in the first plurality of data subcarriers to a null value, and set at least one data subcarrier in the subset of data subcarriers in the second plurality of data subcarriers to the null value.
 19. An apparatus according to claim 17, wherein the physical layer processing unit is configured to: set at least one data subcarrier in the subset of data subcarriers in the first plurality of data subcarriers to a non-zero value, and set at least one data subcarrier in the subset of data subcarriers in the second plurality of data subcarriers to the non-zero value.
 20. An apparatus according to claim 17, wherein the physical layer processing unit is configured to: map the signal field to a third plurality of data subcarriers corresponding to a third frequency portion of the OFDM symbol, set a subset of data subcarriers in the third plurality of data subcarriers to one or more predetermined values, map the signal field to a fourth plurality of subcarriers corresponding to a fourth frequency portion of the OFDM symbol, set a subset of subcarriers in the fourth plurality of subcarriers to one or more predetermined values, and set guard tones and pilot tones in the third frequency portion and the fourth frequency portion. 